This invention relates to an electrolytic apparatus used for producing nitrogen trifluoride gas by a molten salt electrolysis. In particular, this invention relates to a structural configuration of an electrolytic cell, which leads to a more efficient production of nitrogen trifluoride gas. The invention further relates to electrolytic cells and to methods and systems which are useful for efficiently producing nitrogen trifluoride gas.
There is currently a large and growing requirement of nitrogen trifluoride (NF3) gas for use in semiconductor manufacturing. Nitrogen trifluoride may be used, for example, as an etchant or chamber cleaning gas. Demand for these uses has significantly increased in recent times. In such applications, a nitrogen trifluoride gas of high purity and having a carbon tetrafluoride (CF4) by-product content as low as possible is desired.
NF3 gas can be manufactured by various methods. Among them, a molten salt electrolysis gives good yield and is suitable for mass production as compared with other methods and therefore, is regarded as a useful commercial process. In particular, for the purpose of producing a highly pure NF3 gas containing only a small amount of CF4, the molten salt electrolysis method can produce NF3, at the lowest cost. In general, according to a process for producing NF3 gas by a molten salt electrolysis, exemplary suitable molten salt baths comprise acidic ammonium fluoride, NH4F.HF systems derived from ammonium fluoride and hydrogen fluoride, or KF.NH4F.HF systems produced by adding acidic potassium fluoride or potassium fluoride to the NH4F.HF system.
In the process of manufacturing NF3 gas, NF3 gas and nitrogen (N2) gas are generated at the anode while hydrogen (H2) gas is generated at the cathode. That is, gas generating reactions occur at the both electrodes. When NF3 gas generated at anode is mixed with H2 gas generated at cathode, there is a risk of explosion and, therefore, it is necessary to minimize the likelihood of H2 mixing with NF3 at the anode in amounts that may cause explosion. Moreover, the presence of H2 in the anode will lead to other unwanted reactions, such as, for example, with F2 and NF3 to form HF and N2, which decreases the efficiency of the cell and the productivity of NF3.
The geometry of prior art electrolytic cells for generating NF3 can contribute to the problem of H2 migration to the anode by limiting the circulation of formed gases and liquid electrolyte around the cathode and the anode. The longer it takes for the formed gas to be removed from the cell, the more likely that H2 migration to the anode will occur.
Accordingly, there remains a need for a safe and efficient manufacturing apparatus and method for the continuous production and generation of NF3 while producing substantially no unwanted by-products.